Try alternative way of parallelizing coverage

Author: jobh

hypothesis-python/.coveragerc

@@ -1,5 +1,7 @@
 [run]
+parallel = True
 branch = True
+source = hypothesis
 omit =
     **/_hypothesis_ftz_detector.py
     **/_hypothesis_pytestplugin.py

hypothesis-python/docs/strategies.rst

@@ -212,6 +212,26 @@ loading our pytest plugin from your ``conftest.py`` instead::
     echo "pytest_plugins = ['hypothesis.extra.pytestplugin']\n" > tests/conftest.py
     pytest -p "no:hypothesispytest" ...
 
+Another alternative, which we in fact use in our CI self-tests because it works
+well also with parallel tests, is to automatically start coverage early for all
+new processes if an environment variable is set.
+This automatic starting is set up by the PyPi package :pypi:`coverage_enable_subprocess`.
+
+This means all configuration must be done in ``.coveragerc``, and not on the
+command line::
+
+    [run]
+    parallel = True
+    source = ...
+
+Then, set the relevant environment variable and run normally::
+
+    python -m pip install coverage_enable_subprocess
+    export COVERAGE_PROCESS_START=$PATH/.coveragerc
+    pytest [-n auto] ...
+    coverage combine
+    coverage report
+
 
 .. _alternative-backends:
 

hypothesis-python/tox.ini

@@ -218,20 +218,29 @@ allowlist_externals =
 setenv=
     PYTHONWARNDEFAULTENCODING=1
     HYPOTHESIS_INTERNAL_COVERAGE=true
+    COVERAGE_PROCESS_START=.coveragerc
 commands_pre =
     rm -f branch-check*
     pip install .[zoneinfo]
+    pip install coverage_enable_subprocess
+commands_post =
+    pip uninstall -y coverage_enable_subprocess
 # Produce a coverage report even if the test suite fails.
 # (The tox task will still count as failed.)
 ignore_errors = true
+# We've had problems correctly measuring coverage using pytest-cov when running
+# in parallel, so instead we start coverage implicitly on all (sub-)processes by
+# way of the coverage_enable_subprocesses installation. This requires all options
+# to be set in .coveragerc (including source), but that's ok as it is overridden
+# by --cov=... in conjecture-coverage.
 commands =
-    python -bb -X dev -m coverage run --rcfile=.coveragerc --source=hypothesis -m pytest -n0 --ff {posargs} \
+    python -bb -X dev -m pytest -n auto --ff {posargs} \
         tests/cover tests/conjecture tests/datetime tests/numpy tests/pandas tests/lark \
         tests/redis tests/dpcontracts tests/codemods tests/typing_extensions tests/patching tests/test_annotated_types.py
+    python -m coverage combine
     python -m coverage report
     python scripts/validate_branch_check.py
 
-
 [testenv:conjecture-coverage]
 deps =
     -r../requirements/coverage.txt